1. Technical Field
The present invention relates to a drive unit for a switching element, the drive unit driving a voltage-controlled switching element.
2. Related Art
When gate charge is provided to a switching element, the speed of providing the gate charge is controlled. This is a well-known technique as disclosed in a patent document JP-B-3067448. Specifically, in this technique, the speed of providing gate charge is controlled in a semiconductor switching element (IGBT (insulated gate bipolar transistor)) when the switching element is turned off under the conditions where large current flows across the collector and the emitter of the switching element. More specifically, the switching element is provided with a sensing terminal that outputs minute current having a correlation with the collector current that flows through the switching element. The sensing terminal is connected to the emitter of the switching element via a sensing resistor. The gate of the switching element is connected to the emitter via a resistor and a MOSFET (metal oxide semiconductor field-effect transistor). The MOSFET has a gate connected to a connecting point between the sensing terminal and the sensing resistor.
In the above configuration, the MOSFET is turned on when a large current flows through the switching element and thus the potential difference (sensing voltage) across the sensing resistor becomes high. Thus, electric charge is provided into the gate of the switching element to turn off the switching element. In this case, the resistor provided between the gate and the emitter of the switching element can reduce the speed of providing gate charge (hereinafter “the speed of providing gate charge” is referred to as a “charge rate”). Accordingly, the switching element is prevented from being alternately turned on and off due to the turn-off operation of the switching element. In this way, the reliability of the switching element is ensured.
The technique of controlling charge rate includes so-called active gate control. Under the active gate control, when the drive state of a switching element is switched from an on-state to an off-state, or vice versa, the charge rate is changed to achieve the switching. This technique has a purpose of reducing surge voltage or switching loss caused in switching the drive state of a switching element.
The inventors of the present invention paid attention to the phenomenon that the sensing voltage drastically increases when the drive state of a switching element is switched from an on-state to an off-state, or vice versa, and arrived at a conclusion of using the process of changing the charge rate, which is based on the comparison of the sensing voltage or the rate of change of the sensing voltage with a specified value. However, use of this process involves problems as set forth below.
Mass-produced switching elements have individual differences. The individual differences may lead to differences in the behavior of the sensing voltage between the switching elements when the drive state of the switching elements is switched. If a common specified value is used for the individual mass-produced switching elements for the comparison mentioned above, the difference in the behavior of the sensing voltage between the switching elements may create inconveniences. For example, due to the use of a common specified value, the timing of changing the charge rate may be deviated from an appropriate timing of reducing surge voltage or switching loss. If inconveniences such as the above are created, surge voltage or switching loss may problematically increase.